The invention relates to a method in connection with a pipe grate for a fluidized bed boiler and to a pipe grate.
A grate assembly for a fluidized bed boiler comprising parallel sparge pipes or the like is known e.g. from Finnish publication print 98405, to which corresponds U.S. Pat. No. 5,743,197, as well as from Finnish patent application 961653. In a way typical for such pipe grates, fluidizing air is supplied through cooled sparge pipes so that it is discharged upwards from nozzles located at determined intervals in the longitudinal direction of the sparge pipes, to effect fluidization. The fluidizing air also constitutes the combustion air to effect combustion in a fuel admixed to the fluidized bed material.
It is precisely variations in the fuels that have caused a problem in the design of fluidized bed boilers that the surface area of the grate at the bottom of the fluidized bed boiler, i.e. the horizontal cross-sectional area of the fluidized bed boiler must be dimensioned according to the poorest fuel and a full load. Thus, the horizontal cross-sectional area is too large when the heating value of the fuel is better than with the fuel used for the dimensioning. Similarly, the area is too large with partial loads. An unnecessarily large cross-sectional area will result in the use of extra circulation gas to control the temperature of the bed with dry fuels. Also, the minimum load of the boiler is determined according to the cross-section, because if the load is small, the temperature of the bed will decrease to a level which is too low.
Attempts have been made to solve the above-presented drawbacks in fluidized bed boilers equipped with a so-called wind box in such a way that the wind box placed under the grate is divided in two parts, for example by dividing it in two halves by the middle or by making, in a way, two boxes within each other, wherein the cross-section of the bottom, or the grate, can be divided into a central area and an edge zone. This structural solution is expensive, and the separate wind boxes require air measurements and adjustments of their own. For this reason, divided wind boxes are eliminated in new fluidized bed boilers based on a wind box.
Finnish patent application 970559, to which corresponds international publication WO 95/26483, presents a method for removing fluidized zones in connection with a PFBC power plant for the purpose of controlling the active heat transfer area of pipes in a steam generator. This is accomplished with shelf-like baffle plates which are moved in the vicinity of the walls of the combustion chamber, above the nozzles supplying fluidizing air. It is mentioned in the application that to improve the blocking effect of the baffle plates, it is possible to close the air supply from the fluidizing nozzles underneath. In the structure presented in the application, the nozzles are separate fluidizing nozzles connected to pressurized air in a pressure vessel, and it does not mention how the air supply through certain nozzles can be turned off. The shelf-like baffle plate which is primarily used in the adjustment is a massive structure which requires reconstruction work in the walls of the furnace.
It is an aim of the invention to eliminate the above-mentioned drawbacks and to present a method in connection with the grate assembly of a fluidized bed boiler, whereby the area of the air supply can be changed in a simple manner without operations in the space above the grate and whereby the adjustment can be effected more precisely than with wind box solutions. To achieve these aims, the method is characterized as described below. In at least some of the pipes of the pipe grate, the supply of fluidizing air to the pipe is controlled in a pipe-specific manner. The pipe grate, in turn, is also characterized as described below. In the pipe grate, at least some of the pipes in the grate have a pipe-specific control means acting on at least part of the supply of air to the pipes. The control means is used to shut off the supply of fluidizing air to the pipe, and such control means are preferably provided in at least the outermost pipes of the grate, i.e. the pipes close to the side wall of the boiler. The control means can also be a partition wall inside the pipe, dividing the pipe in the longitudinal direction in two different sections each with a separate air supply.
According to further advantageous embodiments of the invention, the grate can be divided, for example, in zones extending parallel or perpendicular to the pipes.